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(11) | EP 0 212 803 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Solid-state image sensor |
| (57) The sensor uses a Charge Sweep Device as a vertical transfer device (3) and comprises
a plurality of pixels (10) each of which is formed on a single photo-electro transforming
element (1) and a single transfer gate (4) for transferring a signal charge from the
photo-electro transforming element into the Charge Sweep Device (3). The transfer
gate (4) is equal to, or larger than, the photo-electro transforming element (1) in
width along the direction of charge transfer in the Charge Sweep Device (3). |
BACKGROUND OF THE INVENTION
Field of the Invention
Description of the Prior Art
[0002] Fig. 1 is a diagram showing a layout of one pixel of an image sensor employing a Charge Sweep Device, disclosed in, e.g., Kimata et al., "A 480 x 400 Element Image Sensor with a Charge Sweep Device", ISSCC Digest of Technical Papers, Feb. 1985, pp. 100-101 and in Japan Television Society Technical Report No. TEVS 101-6, ED 841, 1985, by Kimata et al.
[0003] A solid-state image sensor comprises a photodiode 1 for converting a given optical signal into a signal charge, a transfer gate 4 having a surface channel for selectively reading the signal from the photodiode 1, and a transfer channel 3 composed of a buried channel forming a path for transferring the signal charge from the transfer gate 4. The charge transfer operation of the transfer gate 4 and the transfer channel 3 is controlled according to a voltage level applied from a scanning line 5 to a gate electrode 2 through a contact hole 6. Namely, one pixel in the solid-state image sensor is composed of one photodiode and one gate. The gate electrode 2 applies the same voltage to the transfer channel 3 and the transfer gate 4.
[0004] Fig. 2 is a diagram schematically illustrating potential formed on the cross section taken from the line A-A' in Fig. 1, wherein Fig. 2 (a) shows a potential state at the time when a signal charge is stored; and Fig. 2 (b) shows a potential state at the time of signal reading. In Fig. 2 (a), region I corresponds to the portion of the photodiode 1, region II corresponds to the portion of the transfer gate, and region III corresponds to the buried transfer channel 3. or transfer device (CSD) portion. The operation will be hereinafter described with reference to Figs. 1, 2 (a) and 2 (b).
[0005] First, the signal charge storing operation will be described. In this operation, the potential applied to the gate electrode 2 changes repeatedly between the "H" level and the "L" level. Since the transfer gate electrode 4 shares the gate electrode 2 with the CSD gate electrode, the potential of the transfer gate 4 changes between 0 T(H) and φT (L), and the potential of the CSD gate 3 changes between 0C (H) and φC (L), respectively, in response to the potential applied to the gate electrode 2. If the transfer gate 4 operates in the accumulation mode in this period, 0T (H) may be equal to φT (L). Now, since the transfer gate 4 is composed of a surface channel and the CSD 3 is composed of a buried channel, the potential well formed in the CSD 3 becomes deeper than that of the transfer gate 4. A potential well formed below the photodiode 1 is deeper than the potential well below the transfer gate 4. Therefore, in this condition, the signal charge QsSig detected by the photodiode 1 is not transferred but stored in the potential well below the photodiode 1.
[0006] Next, the signal-reading operation will be described. In this operation, the potential applied to the gate electrode 2 becomes the "HH" level, which is further higher than the "H" level. Accordingly, the potential wells formed below the transfer gate 4 and the CSD 3 become deeper than those in the storing opeation shown in Fig. 2 (a), and the signal charge Qsig stored in the photodiode 1 is read out to the CSD 3. The read signal charge is transferred in the horizontal direction along the transfer channel of the CSD 3 to be read out successively.
[0007] Fig. 3 is a cross sectional structure diagram taken from the line B-B' in Fig. 1. In Fig. 3, an impurity region 8 of a second conductivity type for forming a buried channel serving as the charge transfer channel is provided on a semiconductor substrate 7 of a first conductivity type. An impurity diffusion layer 9 for cell isolation having high impurity density of the first conductivity type formed by, e.g. the ion implantation and a cell isolation oxide film 10 formed by, e.g. the selective oxidation process are provided in order to electrically isolate cells adjacent to one another. As shown in Fig. 3, the impurities from the impurity region 9 for cell isolation having high impurity density diffuse into a channel region 8. In the case of the selective oxidation process, the oxide film 10 for cell isolation is formed after the formation of the impurity region 9. During thermal processing in the formation of the oxide film 10, the impurity diffuses from the impurity region 9 into the transfer channel 8; this may be the reason of the foregoings. In this case, if the channel width of the CSD becomes narrower, the substantial impurity density of the channel 8 decreases under the influence of the impurity region 9, resulting in a phenomenon that the potential well in the transfer channel becomes narrower (so called narrow channel effect).
[0008] Fig. 4 schematically illustrates the potential state on the cross section taken along the line C-C' in Fig. 1. In Fig. 4, the region IV represents a transfer channel region connected to the transfer gate. In the region IV, a transfer gate is formed on the one side of the transfer channel (CSD) and the impurity region 9 having high impurity density shown in Fig. 4 is formed on the other side thereof. Therefore, the narrow channel effect in the transfer channel with impurity regions of high impurity density formed on the both sides thereof is stronger than the narrow channel effect in the transfer channel region connected to the transfer gate. Consequently, a potential well is formed in the transfer channel region IV connected to the transfer gate 4, in which the charge QB-is stored. Therefore, in signal charge transfer operation in the above described structure, the signal charge is transferred with the charge QB remaining in the potential well, resulting in incomplete transfer of the charge which reduces the transfer efficiency.
[0009] In addition, there is a possibility of noise being provided by the fluctuation of the residual charge QB.
[0010] With respect to the basic structure and operation of a conventional solid-state image sensor, see, Yamawaki et al., "A 1/2 FORMAT COLOR IMAGE SENSOR with 485 x 510 PIXELS", SPSE & IGC Electronic Imaging '85, ADVANCE PRINTING OF PAPER SUMMARIES, Oct. 1985, pp.91-94, which discloses a color image sensor using a Charge Sweep Device as a vertical charge transfer device, as well as Kimata et al., "A 480 x 400 Element Image Sensor with a Charge Sweep Device", ISSCC Digest of Technical Papers, Feb, 1985, pp, 100-101, which is referred to with respect to Fig. 1.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a solid-state image sensor which reduces the above described problems, prevents the formation of the potential well in the transfer channel and enables a more complete transfer of the signal charge.
[0012] In the image sensor according to the present invention, the width of the transfer gate alonq ,the direction of the signal charge transfer is made equal to or wider than the width of a photo-electro transforming element such as a photodiode along that direction.
[0013] By widening the width of the transfer gate, the narrow channel effects are restrained uniformly along the direction of the charge transfer in the charge transfer channel thereby preventing a potential well from being built-in the transfer channel.
[0014] These and other object and features of the present invention will be apparent from the detailed description with reference to the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015]
Fig. 1 is a plan view showing a structure of one pixel of an image sensor according to a conventional CSD system.
Fig. 2 is a cross sectional view taken along the line A-A' in Fig. 1 showing the potential state formed at the time of charge storage operation and charge read out operation.
Fig. 3 is a cross-sectional view taken along the line B-B' in Fig. 1.
Fig. 4 is a cross sectional view taken along the line C-C' in Fig. 1 showing the potential state formed thereon.
Fig. 5 is a plan view showing a structure of a solid-state image sensor according to one embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] Fig. 5 is a plan view showing a layout of one pixel of the solid-state image sensor according to one embodiment of the present embodiment, which corresponds to the conventional one in Fig. 1. Corresponding parts are denoted by the same reference numerals. The pixels may be arranged in a matrix of rows and columns, the rows may extend in the vertical or field of direction of the image area. As may be seen from Fig. 5, the width WT of the transfer gate 4 along the direction of the charge transfer, which may be in the row direction, is made equal to the width WP of a photo-electro transforming element such as a photodiode along the direction of the charge tranfer. Generally, the width WI of the isolation for electrically isolating the adjacent photo-electro transforming elements (e.g. photodiode) 1 is Wi << Wp, so that almost all of the channel 3 of the CSD is adjacent to the transfer gate 4.
[0017] The operation of the present invention will be hereinafter described. The signal charge storage operation and the charge read out operation are completely the same as the conventional operations. However, in the structure of the transfer gate according to the present invention, since WP = WT >> WI as described above, the narrow channel effect shown in Fig. 3 is reduced, and the effect becomes almost uniform along the charge transfer direction in the transfer channel 3. In the isolation W part, isolation layers are formed on both sides of the transfer channel 3, causing the narrow channel effect. However, the potential well as shown in Fig. 4 is not formed, since the length WI is quite short and the region corresponds to the contacting part of two gates (gate electrodes between adjacent transfer devices), thereby causing the fringe field effect. Consequently, the perfect transfer of the charge is carried out to enhance the transfer efficiency and to reduce the noise.
[0018] Although in the above described embodiment, the oxide film for cell isolation is formed by the selective oxidation process, other method for forming oxide films may be utilized to obtain the same effect as the above described embodiment.
[0019] In addition, although in the above described embodiments, the solid-state image sensor using CSD system is described, the present invention may be similarly adaptable to a conventional IL-CCD (interline CCD) system provided that it comprises one pixel formed of one gate.
[0020] Although a photodiode is employed as a photo-electro transforming element in the above described embodiment, a solid-state image sensor employing the MIS (metal-insulator-semiconductor) structure or a Schottky barrier diode may be available to obtain the same effect.
[0021] Although the width W of the transfer gate is equal to the width Wp of the photo-electro transforming element in the above described embodiment, the same effect can be obtained in the case where WT > Wp.
[0022] As described above, according to the present invention, a solid-state image sensor capable of preventing the formation of the potential well in the transfer channel having high transfer efficiency and low noise can be obtained, since the width of the transfer gate along the direction of charge transfer is made equal to or wider than the width of the photo-electro transforming element along the direction of the charge transfer.
[0023] Although the present invention has been described with reference to a specific embodiment, it should be understood that the above embodiment is an illustrative preferred embodiment and that modifications may be made without departing from the scope of the invention as defined in the appended claims.
1. A solid-state image sensor comprising a plurality of pixels (10), arranged in a
matrix of rows and columns, each converting a supplied optical signal into a signal
charge, and transfer means (3) for transferring signal charge from pixels among said
plurality of pixels which are arranged in the same row in the direction of the row:
each of said plurality of pixels comprising
a photo-electro transforming element (1) having a width Wp in said direction of the row for transforming a given optical signal into a signal charge, and
a transfer gate (4) having a width WT in said direction of the row, for selectively reading the signal charge from said photo-electro transforming element (1) to be transferred to said transfer means, characterised in that the said width WT of said transfer gate (4) is greater than, or equal to, said width Wp of said photo-electro transforming element (1).
2. A sensor according to claim 1 wherein the said row extends in the vertical direction
of the image area.
3. A solid-state image sensor according to claim 1 or 2, wherein said photo-electrode
transforming element (1) is a photodiode.
4. A solid-state image sensor according to claim 1 or 2, wherein said photo-electro
transforming element (1) is a Schottky diode.
5. A solid-state image sensor according to claim 1 or 2, wherein said photo-electro
transforming element (1) is a metal-insulator-semiconductor device.
6. A solid-state image sensor according to any preceding claim, wherein said tranfer
means is a Charge Sweep Device.